The present invention relates to a ceramic glow plug for starting a diesel engine, and more particularly to a ceramic heater of such a glow plug.
It is well known generally that a diesel engine is hard to start at low temperatures. In order to resolve this problem, it has been the practice to provide a glow plug in the engine's cylinders or in auxiliary combustion chambers thereof for increasing the temperature of the cylinders or the auxiliary combustion chambers. In order to provide quick starting, a glow plug has to have a rapid heating characteristic. Further, due to the recent tendency to use glow plugs not only for starting but also during the engine operation to stabilize the fuel combustion in the cylinder after the engine has started, improvements in the electrical and chemical durabilities of glow plugs have been demanded.
In order to meet this demand, a ceramic glow plug of a rapid-heating type has been developed which includes a ceramic heater composed of a sintered ceramic body and a tungsten heating wire embedded therein. Tungsten is used because it has a high melting point and is heat durable.
FIG. 1 shows, in partial cross section, an example of such a conventional ceramic glow plug. In FIG. 1, a ceramic glow plug 1 includes a ceramic heater 3 constituted by a sintered ceramic body of Si.sub.3 N.sub.4 and a heating wire 2 in the form of a coil embedded therein. The ceramic heater 3 is fixedly secured to one end of a metal sheath 4 to which one end of the heating wire 2 is connected. The metal sheath 4, which is connected to a mounting shell 5, functions as a negative electrode. The other end of the heating wire 2 is connected to a center conductor 6 supported in the shell 5 and electrically insulated therefrom. The center conductor 6 functions as a negative electrode.
A portion 3' of the ceramic heater 3 of the ceramic glow plug 1 in which the heating coil 2 is embedded protrudes from the end of the metal sheath 4 by a distance l. The distance l is typically 12 to 15 mm for a glow plug operated by a 12 V battery. This distance should be as short as possible for reasons of mechanical strength against mechanical shock applied to the ceramic glow plug. In the conventional ceramic glow plug, however, it is impossible to shorten the distance l because, if the ceramic heater 3 is merely pushed into the metal sheath 4 as shown in FIG. 2 to reduce the distance l of the exposed portion 3' thereof to l.sub.1, the portion of the heating wire of the ceramic heater 3 corresponding to a distance l.sub.2 is covered by the metal sheath 4, resulting in a reduced heating efficiency of the heating coil. In addition, the heating wire in the metal sheath may overheat, causing a soldering material used to connect the ceramic heater 3 to the metal sheath 4 to be melted.
For a ceramic glow plug used with a 24 V battery, in order to obtain the same temperaure of the ceramic heater as that of the 12 V ceramic heater, the resistance of the heating coil must be about four times that of the heating coil for the 12 V battery. If such a large resistance is obtained by reducing the diameter of the heating wire, which is usually about 0.2 mm for the 12 V battery, the diameter must be about 0.1 to 0.13 mm for the 24 V battery. When such a thin heating wire is embedded in a ceramic body, it cannot withstand the unavoidable torsional stresses applied thereto due to the difference in thermal expansion coefficients between the ceramic and the heating wire. On the other hand, if the increase of the resistance is obtained by using a longer wire, the distance l of the ceramic heater portion protruding from the end of the metal sheath is necessarily increased. Thus, these approaches are not usable in view of the mechanical strength of the ceramic glow plug.